Buried reservoirs of pre-stressed concrete



Ju y 1956 E. FREYSSINET BURIED RESERVOIRS OF PRE-STRESSED CONCRETE FiledSept. 22, 1952 2 Sheets-Sheet 1 INVENTOR 3 Ma 6'4 M fi0ai7n July 24,1956 Filed Sept. 22, 1952 E. FREYSSINET BURIED RESERVOIRS OFPRE-STRESSEID CONCRETE 2 Sheets-Sheet 2 WORN/5V5 BURIED RESERVOIRS OFPRE-STRESSED CONCRETE Eugene Freyssinet, Neuilly-sur-Seine, FranceApplication September 22, 1952, Serial No. 310,893

Claims priority, application France October 4-, H51

10 Claims. (Cl. 61-.5)

The present invention has for its object a buried reservoir ofpro-stressed concrete, in which the pro-stressing is obtained byutilizing the weight of the masses of movable earth under which thereservoir is buried.

It covers the method of building this reservoir and the apparatusintended for carrying out this method in practice.

The reservoir is composed of two superposed parts, joined on a commonhorizontal plane.

The upper part which supports the earth covering the reservoir is formedby a cupola, preferably of revolution shape, made of relatively thinconcrete and having advantageously a constant thickness along sectionsthrough horizontal planes, which in the case of a cupola of revolutionare parallels. This concrete may be reinforced with a simple very lightcheck-pattern reinforcement.

The shape of this upper part is defined in such a way that, at everypoint, the compressions due to the load of earth create, in theconcrete, unitary compression stresses of the same order in alldirections. cupola of revolution, in particular, it is known that if Pis the pressure on the cupola (the difference between the internal andexternal pressures), the formula of Laplace gives the relation:

N1 and N2 being pressures per meter of length along the meridians andthe parallels and R1 and R2 the main radii of curvature. Moreover, thenecessary balance of loads and reactions supplies on each parallel asecond equation f(N1, N2, P)=:0. The local thicknesses 6 can always beso defined that the local pressures equal to and have such a value as isdesired.

The values of T, and therefore of the prestresses, thus determined arerather badly defined, unless very particular care is employed in thefilling up or packing with earth.

in the method according to the invention, after properly packing withearth which is preferably hydraulically tamped and which may comprisesand or gravel towards the bottom, one avails himself, in order to loadthe concrete the upper part, of all the weight of the earth covering thereservoir together with the frictions between this earth and theneighbouring ground and, furthermore, an accurately predetermined valueis given to this load.

To this end there is arranged, between the upper part and sufficientlyresistant and preferably pro-stressed foundation a joint comprisingjacks, advantageously of the flat type, composed of inflatable metallicbags of the type described by the applicant in his U. S. Letters PatentNo. 2,226,201 then, after completion of the upper part and constructionof the packing, a pressure is exerted between the foundation and thespringing of the upper part along a suitable direction chosen in orderto effect a In the case of a 0 Patented July 24, 1956 shortening bycompression of the perimeter of the base of the said upper part and araising of this part covered by its packing, the result being to load ina definite manner the said part not only with the whole weight of thesaid packing but also with the friction of this packing against theneighbouring ground.

The effect of the pro-stressing thus obtained is made permanent in thereservoir finally constructed, by immobilising the upper part afterraising, for example by means of wedges introduced into the jointbetween the jacks as the raising takes place, the jacks being adapted tobe then withdrawn, or by blocking the jacks, which is effected byinjecting a hardening liquid to operate them.

The invention is illustrated by way of example in the accompanyingdrawings, wherein:

Fig. l is a diagrammatic view in partial vertical section, of one mannerof constructing a reservoir according to the invention.

Fig. 2 is a view, similar to Fig. 1 but on a larger scale, showing thedetail of the joint between the upper spherical vault and thepre-stressed annular foundation.

Figs. 3, 4 and 5 show details.

in the embodiment shown in the drawings both the upper part A and thelower part B of the reservoir have shapes of revolution about a commonvertical axis XX. The sections in Figs. 1 and 2, which are only partialviews, are therefore taken along a meridian plane. The two parts havethe forms of spherical caps, but this form is not limitative. The lowerpart B rests on the soil of the bottom of the excavation of which theslope is indicated at l, as well as on an annular foundation 2. Thesprings of the upper spherical vault A provided with the shaft 3 foraccess rest on this foundation by means of a joint of which the detailsWill be given hereinafter.

The foundation 2 may be of prestressed concrete. An annular prestresscan be conveniently effected with the help of jacks (preferably of theso-called flat type according to the aforesaid patents of the applicant)disposed in 2, 3 or 4 radial joints.

Fig. 2 assumes that one of these joints is arranged in the plane of thedrawing and comprises four fiat jacks 4 each comprising two parallelcircular plates connected by a toric flange 5. The foundation isfurthermore prestressed transversely by looped cables 6. These cables,covered by a lubricant and by lapping to avoid adherence to theconcrete, are stretched after the grip by the con crete and enclosed attheir ends, for example by wedge anchorings as described in the UnitedStates Patents Nos. 2,270,240; 2,618,147 and 2,686,963.

The detail of the loops which each of them forms is shown in Fig. 3,which represents a part of the foundation 2 developed. it the soil isbad the foundation can be enlarged or reinforced by light piles of sand,wood or concrete. The spherical vault A is constructed on the annularfoundation 2 or more exactly on an annular bed of fiat jacks 7 separatedby keys 8, which bed lies on the foundation. Each jack is composed inthe example illustrated of two circular parallel steel plates connectedat their edges by a toric flange 5 provided with an injection appendage.Each jack is preferabiy enclosed in a prismatic mass 7a of concrete. Thekeys 3 placed in the spaces between the jacks have a cuniform verticalsection so as to form wedges which are buried, as will be hereinafterdescribed, below the cupola in accordance with the inflation of thejacks, the surfaces id of which the slant corresponds to that of thesaid wedges being formed on the supporting surface of the spring of thespherical vault (see the diagrammatic part in perspective of Fig. 4) inorder to co-operate with the wedges. Once the spherical vault iscompleted, the excavation is filled in and the spherical vault is loadedwith the packing up to the prescribed height. After formation andhydraulic tamping of the packing, the jacks 7 are inflated so as toexert a pressure between the foundation 2 and the spring of the vault.The surface of this foundation and the supporting surface of the springsof the vault are so directed that the action of the jacks produces ashortening of the perimeter of the base of the vault equal to thecompression to be obtained in the concrete. For example, if on aspherical vault having a radius of two meters, it is desired to effect ashortening of that is to say a reduction in radius of 5 mm., with astroke of the jacks of one centimeter, it is necessary that the line ofaction of the latter should be inclined at /2.

Furthermore the jacks 7 can be arranged, not directly between the vaultand the foundation 2, but between the vault and pivotal elements 11(Fig. 5) supported by ball shaped projections 12 on the foundation 2which renders the deformations of the base parallel of the vaultindependent of the stroke of the jacks.

The keys or wedges 8 are buried between the vault and the foundations asthe jacks are inflated. In case of damage to a jack, this allows ofreleasing the pressure in this jack and of replacing it by another jackfrom the interior of the reservoir, by which the whole manipulation iseffected.

The action of the jacks has for its effect not only to producecompressions in the concrete of the vault by shortening its baseperimeter as has been described, but also to raise the vault covered byits packing earths which thus load this vault in a sure and uniform Way.Furthermore the mass of earth raised at the same time by the vault makesfriction with the adjacent ground. The vault is finally loaded not onlywith the total Weight of the earth surmounting it, but with a largeaddition due to the friction which has been described, the wholefurthermore limited to the measurable action of the jacks. In order toillustrate the operation, an example is given; a packing having a heightof three meters above the centre voussoir of a cap, a diameter of 25meters and a rise of about 6 meters suffices to cause prestressings ofthe order of 60 kgs./sq. cm. in a shell of 12 cms. thickness, even ifthe piezometric level of the liquid in the reservoir shall rise severalmeters above the top of the latter.

The lower part B may be formed in such a manner as desired. But in areservoir more or less deeply buried, it often arises that there is thenecessity of re sisting sub-pressures. They can however be limited bysurrounding the said lower part by a drainage, for example by a packingof stones comprising a discharge duct at a given level. Preferably thesaid lower part will be formed by a cap which can be spherical as shownin the drawings or of form calculated to give, in the zone of support ofthe upper vault, minimum horizontal reactions. This cap may have forexample a radius decreasing from the centre to the periphery. Preferablyit will rest against the base of the upper vault.

It is possible and advantageous to prestress this cap. It is necessaryfor this so to arrange things that its concrete can shorten itselfelastically despite the support on the bottom of the excavation.Commencement may be made by sweeping-up according to the form chosen forthe meridian of the cap, a concrete layer 15 for neatness which willcause all irregularity of form to disappear especially towards thefoundation and which is packed with a plastic body such as bitumen. Thelayer of concrete 16, to be prestressed, is run onto the bitumen Whilstarranging an annular joint 17 near the base of the upper vault. In thisjoint, extending along a parallel of the cap, there is inserted a bagjack formed by a tube of flattened sheet metal 18 well supported bymortar packings against the concrete on the two faces of the joint. Thistube is inflated with a liquid (water or oil) so as to compress theconcrete 16 forming the operative part of the lower cap, after which,for this 4 liquid a body is substituted which is capable of forming aconnection: cement grout, synthetic resin which keeps the jack 18inflated. The fluidtight jack thus formed is completed by a packing ofthe upper part of the remaining joint and an injection of the lowerinaccessible part. The peripheral pressure created on the base of thevault by the action of the jack 18 will have for its effect to deflectthe reaction of the vault on the foundation Allowance will be made forthe construction of the annular foundation 2. For example thisfoundation can be prolonged on the outer side as indicated at 20, by aconcrete covering of the earths before filling in the excavation andplacing the vault under compression. By friction on the ground thisextension 2a of the foundation will provide the necessary additionalreaction.

The invention provides the means for forming prestressed reservoirs by aconstruction which is simple and very economical, the proportion ofprestrcssing being the higher, the thinner the Walls.

The type of reservoir may be adapted to all grounds and all dimensions.It can even be constructed as a subterranean at great depth by boring aWell which will be enlarged at its base like an elephants foot. Into thecavity thus formed, parts of the reservoir will be constructed as hasbeen described, and in order to prestress the upper cap, between it andthe wall of the cavity which overhangs it there can be interposedsandy-clay mud of a certain thickness intended to adjust the pressures.This mud will be compressed between the vault and the wall of the cavitywhen the vault is prestresscd.

What I claim is:

1. In the construction of a buried reservoir having a concrete wall ofgenerally arcuated shape, the convexity of which faces upward, and afoundation supporting said wall through at least a part of the baseperimeter thereof a process of prestressing said concrete Wallcomprising the steps of packing earthlike material on top of said wall,exerting a pressure between said foundation and said part of the baseperimeter of said wall so as to produce a shortening by compression ofsaid perimeter and a raising of said wall covered with said material,and sealing the crack thus produced between said wall and saidfoundation.

2. Process as recited in claim 1, further comprising the step of holdingin a substantially permanent manner said wall in spaced relationshipwith respect to said foundation, whereby said wall remains with itsshortened perimeter and in its raised position, even upon release ofsaid pressure.

3. Process as recited in claim 1, wherein said foundation is made ofconcrete, further comprising the step of prestressing said concretefoundation.

4. A buried reservoir comprising a prestressed concrete Wall ofgenerally arcuated shape, the convexity of which faces upward, afoundation supporting said Wall through at least a part of the baseperimeter thereof, earthlike material packed on top of said wall,pressure exerting means between said foundation and said part of thebase perimeter of said wall, to hold said wall in a raised positionwherein its perimeter is compressed and a crack is produced between saidwall and said foundation, and means sealing said crack.

5. Reservoir as recited in claim 4, wherein the pressure exerting meanscomprises a hollow, inflatable jack inserted between said foundation andsaid part of the wall, and inflating material filling said jack.

6. Reservoir as recited in claim 4, wherein the pressure exerting meanscomprises a wedge-shaped member inserted between said foundation andsaid part of the wall.

7. A buried reservoir comprising a curved pre-stressed wall of generalrevolution shape about a generally vertical axis and of upwardly facingconvexity, a generally annular foundation supporting said wall throughthe base perimeter thereof, earthlike material packed on top of saidwall, a plurality of spaced, pressure exerting means between saidannular foundation and the base of said Wall, acting along inwardlyinclined directions to hold said wall in a raised position wherein itsbase perimeter is compressed and a crack is produced between said walland said foundation, and means sealing said crack.

8. Reservoir as recited in claim 7, wherein the direction of action ofsaid pressure exerting means lies in a general axial plane betweenvertically upward and horizontally inward.

9. Reservoir as recited in claim 8, wherein the pressure exerting meanscomprise a series of flat, hollow, inflatable jacks circularlydistributed along said annular foundation, said jacks being inclinedrelatively to each other, and an inflating material filling said jacks.

10. Reservoir as recited in claim 8, wherein the pressure 15 exertingmeans comprise a series of wedge-shaped members circularly distributedalong said annular foundation, said members being inclined relatively toeach other and inserted between said foundation and the base of saidwall.

References Cited in the file of this patent UNITED STATES PATENTS903,909 Steiner Nov. 17, 1908 1,301,155 Mueser Apr. 22, 1919 2,226,201Freyssinet Dec. 24, 1940 FOREIGN PATENTS 416,698 Great Britain Sept. 19,1934 977,666 France Nov. 15, 1950

